1. Field of the Invention
This invention relates to an electro-hydraulic control system for providing a hydraulic control pressure to a brake system so as to apply a brake torque to a powered wheel of a vehicle when excessive slippage of the wheel is detected.
2. Discussion of the Prior Art
Traction control systems are known which detect slippage of one powered wheel of a vehicle and in response transfer torque to another wheel, typically on the other side of the vehicle. In one such system, the brake of the slipping wheel is applied when the speed of the slipping wheel exceeds the speed of the wheel on the other side of the axle by a predetermined amount, and the application of the brake causes torque to be transferred to the slower wheel through the differential of the axle. It is to such a system that the present invention applies.
Such a system may be applied to the treadle operated brake retarder system of a heavy vehicle, such as an earth moving vehicle or a semi-truck tractor. In a brake retarder system, a separate treadle is provided which when depressed applies the brakes to only the rear wheels of the vehicle. Such systems are commonly provided so as to slow down the vehicle when descending long grades. In such a system, a single treadle is mechanically connected to operate two valves, one of which is for the left rear wheel and the other of which is for the right rear wheel.
While manual operation is possible with these valves by virtue of the foot operated treadle, each valve can also be provided with a hydraulic pilot pressure port so that each valve may be operated independently of the other by a separate hydraulic pilot pressure for that valve. In such brake valves, the amount of brake torque applied to the wheel is proportional to the hydraulic pressure applied to the pilot port. The present invention relates to a hydraulic control system for providing this pilot pressure to a brake valve, be it the brake valve of a brake retarder system, or some other brake valve which is operated by a hydraulic pressure.
In a traction control system, fast response is desired, so as to begin slowing the slipping wheel as soon as possible with the resultant transfer of torque to the other wheel, so that the vehicle will move forward and not dig itself into a rut. However, smooth and steady operation is also desirable so as to avoid abrupt loads on the brake system, the transmission and the engine of the vehicle. Also, because there are a wide variety of brake systems to which a hydraulic system for controlling the brake system may be applied, it is desirable that the hydraulic system be adaptable to a wide variety of brake systems, as well as to a wide variety of hydraulic pressure supplies, which are typically provided as part of the vehicle.